Method of handling a large structure in a reactor building

ABSTRACT

When carrying the large structure out through an aperture portion provided at a roof of a reactor building, a chamber for covering the aperture is provided at the roof so that work of providing the chamber for covering the aperture portion can be performed independent of other work performed in the reactor building. At least a part of the work of using a roof crane, and at least a part of the work of handling fuel assemblies in the reactor building can be performed in parallel with forming the roof aperture.

This is a continuation application of U.S. Ser. No. 09/805,992, filedMar. 15, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to a method of handling a large structurewhen a large component of a nuclear power plant, such as a rectorpressure vessel, a large structure member in a reactor, etc., isreplaced; or decommissioning of the nuclear power plant is conducted.

A nuclear power plant is designed so that it has a sufficient margin toits life time which was required on the occasion of its construction.Further, the life time of the nuclear power plant can be extended byexchanging life-expired components/parts of the plant. In the case wherea large structural component or part to be exchanged is moved into orout of a reactor building, there is sometimes a problem in that the sizeof a conventional entrance provided in the reactor building is toosmall. In this case, it is necessary to provide an aperture portion atthe roof of the reactor building, through which the large structure canbe passed, (hereafter, simply referred to as an aperture portion).

Here, Japanese Patent Application Laid-Open Hei 8-262190 discloses arelevant method in which, when a reactor pressure vessel (hereafterreferred to as a RPV) of a large structure, is exchanged, a block of asize such that which can pass the structure through it, is cut out fromthe roof of the reactor building by a cutting machine using a laserbeam, and the cut-out block is lifted up by a crane.

When a large structure such as a RPV is exchanged, fuel assemblies,components, and etc., are moved by using a roof crane in an operatingfloor of a reactor building. Therefore, it is important to reduce thestopping time of a nuclear power plant by adjusting the proceeding ofwork of moving components and work of providing/occluding an apertureportion at the roof of the reactor building.

Generally, a roof of a reactor building has a multilayer structurecomposed of a steel framework of truss structure, which reinforces thereactor building, a deck plate covering the steel framework, and aconcrete layer of 100-300 mm thickness, which is made up by pouringconcrete onto the deck plate. Here, it is an important subject that thework of providing/occluding an aperture portion, which is performed atthe upper position of the operating floor, does not interfere with workof moving fuel assemblies, components, etc.

However, Japanese Patent Application Laid-Open Hei 8-262190 does notspecifically disclose how the work of providing/occluding an apertureportion is performed. Accordingly, by the method disclosed in thisdocument, it is inevitable that the work of providing/occluding anaperture portion interferes with the work of moving fuel assemblies,components, etc. Further, if the work of providing/occluding an apertureportion is performed over a fuel pool in which fuel assemblies arestored, extraneous parts may fall into the fuel pool. Therefore, thework of providing/occluding an aperture portion must be performed afterall fuel assemblies are taken out of the RPV. Consequently, processes ofexchanging a large structure such as a RPV takes a long time.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a method of handlinga large structure when carrying the large structure out/in through anaperture portion provided at the roof of a reactor building, which canreduce the time necessary to carry the large structure out/in.

To achieve the above objective, the present invention provides a methodof handling a large structure in a reactor building in carrying thelarge structure out through an aperture portion provided at a roof ofthe reactor building, wherein an isolation chamber for isolating work ofproviding the aperture portion is installed on the roof before at leasta part of work of handling fuel assemblies and at least a part of workof using a roof crane, and the work of providing the aperture portion isperformed in parallel with at least a part of work of handling fuelassemblies and at least a part of work of using a roof crane.

Further, the present invention provides a method of handling a largestructure in a reactor building in carrying the large structure outthrough an aperture portion provided at a roof of the reactor building,wherein work of carrying the large structure in is performed in a statein that a chamber for covering the aperture portion, which is installedfor performing work of occluding the aperture portion independent ofother work performed in the reactor building, is installed at the roof;and after the work of carrying the large structure in, at least a partof work of using a roof crane in the reactor building is performed inparallel with the work of occluding the aperture portion.

Furthermore, the present invention provides a method of handling a largestructure in a reactor building in carrying the large structure outthrough an aperture portion provided at a roof of the reactor building,the method comprising the steps of: providing a chamber for covering theaperture portion provided at the roof so that work of providing thechamber for covering the aperture portion can be performed independentof other work performed in the reactor building, before at least a partof work of handling fuel assemblies and at least a part of work of usinga roof crane in the reactor building; performing the work of providingthe aperture portion in parallel with at least a part of the work ofhandling fuel assemblies and at least a part of work of using a roofcrane in the reactor building; performing the work of carrying in thelarge structure; and performing work of occluding the aperture portionin parallel with at least a part of the work of handling fuel assembliesand at least a part of work of using a roof crane in the reactorbuilding after the work of carrying the large structure in.

In accordance with the present invention, when work of carrying a largestructure out/in through an aperture portion provided at the roof of areactor building, since work of providing/occluding the aperture portioncan be performed in parallel with a part of handling fuel assemblies anda part of work using a roof crane, the time of total work can beremarkably reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of procedures performed by a method of anembodiment in which the present invention is applied to work ofexchanging a RPV.

FIG. 2 is a schematic vertical cross section of a reactor building inwhich the method shown in FIG. 1 is performed.

FIG. 3 is a schematic vertical cross section of the RPV shown in FIG. 2.

FIG. 4 is a perspective view of a partially notched roof of the reactorbuilding.

FIG. 5(a) is a plan view of the truss structure shown in FIG. 4.

FIG. 5(b) is a cross section at line A—A shown in FIG. 5(a).

FIG. 6 is a schematic vertical cross section of the reactor buildingwhen step S5 a shown in FIG. 1 is finished.

FIG. 7 is a perspective view of the partially notched apertureportion-covering chamber shown in FIG. 6.

FIG. 8 is a schematic vertical cross section of the reactor buildingwhen step S5 b shown in FIG. 1 is finished.

FIG. 9(a) is a schematic vertical cross section of the upper part of thereactor building when step S5 c shown in FIG. 1 is finished.

FIG. 9(b) is a cross section at line H—H shown in FIG. 9(a).

FIG. 10 is a schematic vertical cross section of the upper part of thereactor building when step S5 d shown in FIG. 1 is finished.

FIG. 11 is a schematic vertical cross section of the upper part of thereactor building during the process of step S7 shown in FIG. 1.

FIG. 12 is a schematic vertical cross section of the upper part of thereactor building during the process of step S8 shown in FIG. 1.

FIG. 13 is a schematic perspective view of the situation around thereactor building when step S8 shown in FIG. 1 is finished.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereafter, details of the embodiments according to the present inventionwill be explained with reference to the drawings.

In the embodiments, the present invention is applied to the exchangingof a reactor pressure vessel (RPV) 2 which is one of large structures ina reactor building 24 of a boiling water reactor type nuclear powerplant. FIG. 2 shows a schematic vertical cross section of the reactorbuilding 24. A primary containment vessel (hereafter referred to as aPCV) 10 is installed in the reactor building 24, and RPV 2 is situatedin PCV 10. Also, A reactor shield wall (hereafter referred to as a RSW)3 is situated outside RPV 2. Further, a spent fuel pool 8 and acomponent pool 81 are installed at the side of PCV 10, and a spent fuelrack 9 is provided in the spent fuel pool 8.

Moreover, a steam dryer (hereafter referred to as a dryer) 6 and a steamseparator (hereafter referred to as a separator) 7 are situated in RPV2. A reactor pressure vessel head (hereafter referred to as a RPV head)5 is attached to the upper portion of RPV 2, and a primary containmentvessel head (hereafter referred to as a PCV head) 10 is also attached tothe upper portion of PCV 10. In the embodiments, the work of exchangingthe RPV 2 is performed in the nuclear power plant of the above-describedcomposition.

FIG. 3 shows a schematic vertical cross section of the RPV 2 shown inFIG. 2. A main steam nozzle 11, a feed water nozzle 12, a core spraynozzle 13, a recirculation inlet nozzle 14, and a recirculation outletnozzle 15, are attached to the side wall of RPV 2. Further, pipingsystems such as main steam piping 16, feed water piping 17, core spraypiping 18, and recirculation inlet piping 19, are connected therespective nozzles. RPV 2 is fixed to a RPV pedestal 61 by using basebolts 60. Also, a fuel exchange bellows 23 and a bulk head plate 21 aresituated at the upper part of PCV 10. Furthermore, a housing 62 of acontrol rod drive mechanism (hereafter referred to as a CRD) and ahousing 63 of an internal core monitor (hereafter referred to as an ICM)are situated on the RPV pedestal 61. Also, RSW 3 is fixed to the RPVpedestal 61 by using base bolts 3 a. Moreover, a PCV stabilizer 22 whichis an earthquake-proof support for the PCV 10, and a RPV stabilizer 56which is an earthquake-proof support for the RPV 2, are situated at theupper part of RSW 3.

FIG. 4 is a perspective view showing the structure of a partiallynotched roof of the reactor building 24. A roof truss structure 27 iscomposed of main beams 28, deck plate-support beams 29, and braces 30.Further, a deck plate 26 is situated on the roof truss 27. Furthermore,a concrete layer 25 is formed by pouring concrete onto the deck plate 26and solidifying the poured concrete. Here, reference number 38 aindicates a place at which the aperture portion is provided.

FIG. 5(a) is a plan view showing the composition of the truss structureshown in FIG. 4. The hatched part 38 a indicates a region in which theaperture portion is provided. FIG. 5(b) is a cross section at line A—Ashown in FIG. 5(a). Reference number 32 indicates stud bolts attached tothe main beams 28.

In the following, the work of exchanging RPV 2, which is performed inthis embodiments, will be explained with reference to FIG. 1. FIG. 1shows a flow chart of procedures of exchanging RPV 2, performed in thisembodiment. First, in step S1, parallel-off of a generator is performedin order to performed the word of exchanging RPV 2, and the periodicalinspection of the nuclear power plant is started.

In step S2, work of opening the reactor is performed. In this work ofopening the reactor, processes of RPV head-disassembly for dismountingthe RPV head 5, steam dryer-disassembly for dismounting the steam dryer6, separator-disassembly for dismounting the separator 7, and so forth,are performed. The work of opening the reactor is the critical work tohandling the fuel assemblies 80 in the core. The steam dryer 6 and theseparator 7 are dismounted in every periodical inspection, and arecomponents to be timely exchanged. Therefore, in the work of exchangingRPV 2, the steam dryer 6 and the separator 7 are transferred to thecomponent pool 81 assuming that these components are again used.

In step S3, work of taking all fuel assemblies 80 (not showndiagrammatically) out of the core is performed. In this work of takingall fuel assemblies 80, all the fuel assemblies 80 loaded in the coreare transferred to the spent fuel pool 9. Next, in step S4, the pipingsystems which are connected to the respective nozzles attached to RPV 2,such as the main steam piping 16, the feed water piping 17, the corespray piping 18, the recirculation inlet piping 19, the recirculationoutlet piping 20, etc.; the bulk head plate 21; the fuel exchangebellows 23; and so forth; are disconnected. Further, structure membersaround RPV 2 are decomposed. Components and fittings, related to theabove decomposition work, are transported by a roof crane 85.

In step S5 a, an aperture portion-covering chamber 83 used to performwork at the aperture portion independent of other work in the reactorbuilding 24, is installed under the place 38 a at which the apertureportion is provided. The aperture portion-covering chamber 83 is also anisolation facility for isolating the work at the aperture portion. Inthis step, since the work of installing the chamber 83 is performed at ahigh position, there is fear that extraneous parts fall into the spentfuel pool 8. Accordingly, after the spent fuel pool is covered by aprotection sheet, this step is performed. In other words, this work ofinstalling the chamber 83 is performed so as not to overlap work usingthe spent fuel pool 8., such as the moving of fuel assemblies 80. Inthis way, the work of installing the chamber 83 is completed beforestarting the work of carrying all fuel assemblies outside the reactorbuilding 24, performed instep S6 which will be explained later.

FIG. 6 is a schematic vertical cross section of the reactor building 24,showing the state in the reactor building 24 when step S5 a is finished.In this figure, reference number 83 also indicates the apertureportion-covering chamber, installed under the place 83 a at which theaperture portion is provided, for the work of providing/occluding theaperture portion. Reference number 27 indicates the roof truss structurefor reinforcing the roof of the reactor building 24. The apertureportion-covering chamber 83 is supported by the roof 24 a of the reactorbuilding 24 or the roof truss 27, and is installed above the roof crane85 so as not to interfere with travel motion of the roof crane 85. Also,reference number 34 indicates the operating floor on which the work ofmoving fuel assemblies and components is performed.

Next, the structure of the aperture portion-covering chamber 83 isexplained below with reference to FIG. 7. FIG. 7 is a perspective viewof the partially notched aperture portion-covering chamber 83, showingthe structure of this chamber 83. The aperture portion-covering chamber83 has an airtight structure for preventing dust, water used for thework, etc., generated in the work of providing/occluding the apertureportion, which is performed in this chamber 83, from leaking in thereactor building 24. The floor 83 a of the aperture portion-coveringchamber 83 has an opened and closed portion 83 b in which an opened andclosed door 83 g is provided. The size of the opened and closed portion83 b is a size such that which can pass a large structure in the reactorbuilding 24 through this opened and closed portion 83 b in the openingstate of this portion 83 b.

Further, the floor 83 a and the wall 83 c have the shielding abilitysuch that which can sufficiently satisfy the reference level of skyshineradiation, in order to make it possible that the work of providing theaperture portion, performed in step S5 d which will be explained later,and the work of moving all fuel assemblies 80, can be performed inparallel even if the aperture portion 38 is provided at the roof of thereactor building 24. The shielding ability satisfying the referencelevel of skyshine radiation means that the floor 83 b and the wall 83 chave the thickness required to reduce the quantity of measuredradiation, which is scattered by air, to below the environmentalstandard level. There is a collection sluiceway 86 a situated at a lowerposition of the space in which the roof-cutting work is performed, forcollecting cut fragments or water used for the cutting work (hereafterreferred to as cut fragments, etc.), which are generated in the work ofcutting the roof of the reactor building 24, and preventing these cutfragments, etc., from flying in all directions. The cut fragments, etc.,collected by the collection sluiceway 86 a, are collected into acollection apparatus (not shown diagrammatically). Moreover, an airconditioner (not shown diagrammatically) is situated in the apertureportion-covering chamber 83 so that workers can work in the apertureportion-covering chamber 83. Here, reference numbers 83 d, 83 e, and 83f, indicate a frame, members for attaching the chamber 83 to the roof,and an entrance door, respectively.

Next, in step S5 b, an openable roof protection chamber 84 is installedabove the place 38 a at which the aperture portion is provided. Byinstalling the roof protection chamber 84, when the roof-cutting work isperformed to provide the aperture portion 38, the reactor building 24,and workers and components on the roof can be protected from rain orwind. Further, this roof protection chamber 84 has a role of a shutteralso.

FIG. 8 is a schematic vertical cross section of the reactor buildingwhen the roof protection chamber 84 is installed in step S5 b. This roofprotection chamber 84 can protect the roof from rain, wind, snow, etc.,and an openable shutter 39 is provided at an aperture portion 84 a usedfor carrying a large structure out/in. The roof protection chamber 84has an airtight structure for preventing concrete fragments, dust, workwater used for the work, etc., (hereafter referred to as work water,etc.), generated by the work of providing/occluding the aperture portion84 a, which is performed in this chamber 84, from leaking outside ofthis chamber 84. Further, a collection sluiceway 86 a, collection piping86 b, and a collection apparatus 86, are provided in order to collectthe work water, etc. Furthermore, an air conditioner 87 is situated inthe roof protection chamber 84 so that workers can work in this chamber84.

Next, in step S5 c, reinforcement members are attached to the roof ofthe reactor building 24. In the step of providing the aperture portion38, a part of the main beams 28 for strengthening the roof is cut andremoved. Before the removing of the part of the main beams 28, the restpart of the main beams 28 which are not removed for providing theaperture portion 38 (the part of the main beams 38 which remain afterthe providing of the aperture portion 38), is reinforced.

The method of reinforcing the rest of the main beams 28 is explainedbelow with reference to FIG. 9(a) and FIG. 9(b). FIG. 9(a) shows aschematic vertical cross section of the upper part of the reactorbuilding when the reinforcement members are attached to the roof of thereactor building 24 in step S5 c. Further, FIG. 9(b) shows a crosssection at line H—H shown in FIG. 9(a). In this example, the roof isreinforced by connecting the reinforcement beams 48, situated on theroof, to lower support plates 51 situated under the rest part of themain beams 28 by using reinforcement bolts 49, which remains after theproviding of the aperture portion. This roof-reinforcement work isperformed in the roof protection chamber 84 and the apertureportion-covering chamber 83. Reference number 52 indicates stud boltsfor fixing the reinforcement beams 48 to the roof (the concrete layer).

There is another roof-reinforcement method in which posts for supportingthe roof are additionally situated in the operation floor 34. However,in this method, the motion of the crane 85 is restricted, which in turnmay cause an obstacle to the work of moving fuel assemblies, etc.

Next, in step S5 d, the aperture portion 38 is provided at the place 38a. This providing of the aperture portion 38 is performed by removingthe part of the concrete layer 25, which corresponds to the place of theaperture portion 38, and by cutting and removing the part of the deckplate 26 and the roof truss 27, which is positioned under the cut partof the concrete layer 25. FIG. 10 is a schematic vertical cross sectionof the upper part of the reactor building when the providing of theaperture portion 38 is finished in step S5 d. Since the work in step S5d is performed in the aperture portion-covering chamber 83 and the roofprotection chamber 84 provided on the roof, this work can be performedindependent of (separate from) the work of moving fuel assemblies 80 orcomponents.

Next, in step S6, all fuel assemblies are carried outside the reactorbuilding 24, and are transferred to an external storage facility. Asdescribed above, by performing the providing of the aperture portion 38in the sealed-up and isolated aperture portion-covering chamber 83located over the spent fuel pool 8, it is prevented that extraneousparts fall into the spent fuel pool 8. Accordingly, it is possible tosimultaneously perform the work of transferring all fuel assemblies 80to the external storage facility, which is performed in step S6, and thework of providing the aperture portion 38, which is performed in step 5d.

Next, in step S7, the RPV shield facility 89 is carried inside thereactor building 24 through the aperture portion 38. FIG. 11 is aschematic vertical cross section of the upper part of the reactorbuilding when the carrying-in of the RPV shield facility 89 is performedin step S7. In this step, the shutter 39 of the roof protection chamber84 installed on the roof, and the opened and closed door 83 g of theaperture portion-covering chamber 83, are opened, and the RPV shieldfacility 89 is carried-in by the crane 36 situated outside the reactorbuilding 24. Further, the carried-in RPV shield facility 89 istemporarily laid on the upper part of RSW 3. After the carrying-in ofthe RPV shield facility 89, the shutter 39 of the roof protectionchamber 84 installed on the roof, and the opened and closed door 83 g ofthe aperture portion-covering chamber 83, are closed so as to preventradioactive substances from leaking outside the reactor building 24.

Next, in step S8, RPV 2 put in the RPV shield facility 89 is taken outthrough the aperture portion 38. In this step, the RPV stabilizer 56 andthe base bolts 60 attached to the upper part of RPV 2 are detached, andRPV 2 is lifted up by the crane 36 situated outside the reactor building24. Further, the lifted-up RPV 2 is put in the RPV shield facility 89temporarily laid on RSW 3 by the crane 36. Then, RPV 2 is taken outalong with the RPV.

FIG. 12 is a schematic vertical cross section of the upper part of thereactor building when RPV 2 is taken out along with the RPV in step S8.In this step as well as in step S7, the shutter 39 of the roofprotection chamber 84 installed on the roof, and the opened and closeddoor 83 g of the aperture portion-covering chamber 83, are opened, andthe RPV shield facility 89 and RPV 2 are carried outside the reactorbuilding 24 by the crane 36 situated outside the reactor building 24. Inthis embodiment, after the RPV shield facility 89 and RPV 2 are carriedoutside the reactor building 24 while both the door 83 g and the shutter39 of the roof protection chamber 84 are opened, the door 83 g and theshutter 39 of the roof protection chamber are closed. Also, a method ofsequentially closing the door 83 g and the shutter 39 when the RPVshield facility 89 and RPV 2 are raised up, is possible, in which thedoor 83 g is first closed if the bottom portion of the door 83 g and theshutter 39 is lifted up above the door 83 g, and the shutter 39 is nextclosed if the bottom portion of the door 83 g and the shutter 39 islifted up above the shutter 39.

When the carrying-out of RPV 2 is completed, the shutter 39 of the roofprotection chamber 84 installed on the roof, and the opened and closeddoor 83 g of the aperture portion-covering chamber 83, are closed so asto prevent radioactive substances from leaking outside the reactorbuilding 24. FIG. 13 is a schematic perspective view of the situationaround the reactor building when the carrying-out of the RPV shieldfacility 89 and RPV 2 is finished in step S8. In this way, a series ofprocedures for the work of carrying RPV 2 out is completed.

Next, in step S9, a new RPV is carried-in through the aperture portion38. In this step as well as in step S7, the shutter 39 of the roofprotection chamber 84 installed on the roof, and the door 83 g of theaperture portion-covering chamber 83, are opened, and the new RPV iscarried-in the reactor building 24 by the crane 36 situated outside thereactor building 24. When the carrying-in of the new RPV is finished,the shutter 39 of the roof protection chamber 84 installed on the roofand the door 83 g of the aperture portion-covering chamber 83, areclosed.

Next, in step S10, structures around RPV 2, such as the bulk head plate21, the fuel exchange bellows 23, the piping systems connected to therespective nozzles attached at RPV 2, and so forth, are reconstituted.In this reconstitution work, components and fittings related to thiswork are moved by the roof crane 85.

Next, in step S11 a, the work of occluding the aperture portion 38 isperformed in the aperture portion-covering chamber 83. In this step, therespective parts of the roof truss 27 and the deck plate 26, which havebeen removed in step S5 d, are restored, and the removed part of theconcrete layer 25 is put into the original place. This apertureportion-occluding work is performed in the sealed-up apertureportion-covering chamber 83. Therefore, the work in this step as well asthat in step 5 d can be performed in parallel with the work of carryingfuel assemblies 80 in the reactor building 24, which is performed instep S12 explained later.

Next, in step S11 b, the roof-reinforcement beams 48 and thereinforcement bolts 48, which have been attached in step S5 c, areremoved. Further, in step S11 c, the roof protection chamber 84installed in step S5 b is disassembled and removed.

Next, in step S11 d, the aperture portion-covering chamber 83 installedin step S5 a is disassembled and removed. In this step, since the workof removing the chamber 83 is performed at the roof of the reactorbuilding 24, which is located at a high position, there is fear thatextraneous parts fall into the spent fuel pool 8. Accordingly, after thespent fuel pool is covered by a protection sheet, this step isperformed. In other words, this work of removing the chamber 83 isperformed so as not to overlap work using the spent fuel pool 8, such asthe moving of fuel assemblies 80. In this way, the work of removing thechamber 83 is completed before starting the work of carrying fuelassemblies 80 in, which is performed in step S6 explained later, orafter the carrying-in of fuel assemblies 80 is finished in step 12.

Next, in step S12, fuel assemblies 80 which have been transferred to theexternal storage facility in step S6, are carried-in the spent fuel pool8 in the reactor building. Further, in step S13, fuel assemblies 80 laidin the spent fuel pool 8 are loaded in the core. Furthermore, in stepS14, the separator 7, the dryer 6, the PCV head 4, the RPV head 5, etc.,are reassembled, and the reactor is restarted. In this way, a series ofprocedures for the work of exchanging RPV 2 is completed.

According to the above embodiment, the work of providing the apertureportion 38, which is performed in step S5 d, and the work of occludingthe aperture portion 38, which is performed in step S11 a, can beperformed in parallel with the work of carrying out fuel assemblies,which is performed in step S6, and the work of carrying in fuelassemblies 80, which is performed in step S12, respectively. That is, byproviding the aperture portion-covering chamber 83 at the roof of thereactor building 24 before at least a part of the work of handling fuelassemblies 80, at least a part of the work of handling fuel assemblies80 can be performed in parallel with the work of providing/occluding theaperture portion 38. Thus, the time for the work of exchanging RPV 2 canbe greatly reduced, and this can also reduce the stopping time of thenuclear power plant.

Meanwhile, in the above embodiment, step S5 a can be executed beforestep S4, and step S11 a can be executed before step S10. Under the aboveconditions, the work of providing the aperture portion 38 in step S5 dand the work of occluding the aperture portion 38 in step S11 a can beperformed in parallel with the work of dismounting the structures aroundRPV 2 in step S4 and the work of reconstituting the structures aroundRPV 2 in step S4, respectively. That is, by providing the roofprotection chamber 84 on the roof of the reactor building 24 before atleast a part of the work of using the roof crane 85, at least a part ofthe work of using the roof crane 85 can be performed in parallel withthe work of providing/occluding the aperture portion 38. This also cangreatly reduce the time for the work of exchanging RPV 2. Further, inthe above embodiment, step S5 a can be executed before step S3. Underthe above conditions, the work of providing the aperture portion 38 instep S5 a can be performed in parallel with the work of taking out fuelassemblies in step S3. This also can greatly reduce the time for thework of exchanging RPV 2.

Furthermore, step S5 a can be performed before starting the periodicalinspection of the nuclear power plant, or step S11 d can be performedbefore starting the periodical inspection of the nuclear power plant.Under the above conditions, the work performed in step S5 a or S11 d canbe omitted, which in turn can further reduce the time for the work ofexchanging RPV 2.

What is claimed is:
 1. A method of forming an aperture portion ofreactor building, wherein an isolation chamber is provided to a roof ofsaid reactor building at a position between said roof of said reactorbuilding and a traveling path of an overhead traveling crane in saidreactor building, and an aperture portion for passing a structuretherethrough is formed at a location directly above said isolationchamber of said roof of reactor building.
 2. A method of forming anaperture portion of reactor building as claimed in claim 1, wherein saidisolation chamber is provided with doors which are passable of saidstructure.
 3. A method of forming an aperture portion of reactorbuilding as claimed in claim 1, wherein doors which are passable of saidstructure are provided to said isolation chamber after fixing saidisolation chamber at said roof of said reactor building.
 4. A method offorming an aperture portion of reactor building as claimed in claim 1,wherein said structure is a reactor vessel, and said location of fixingsaid isolation chamber is a portion directly above said reactor vesselin said reactor building.
 5. A method of forming an aperture portion ofreactor building as claimed in claims 1, wherein said isolation chamberhas a structure to prevent dust generated by forming operation of saidaperture portion from diffusing into said reactor building.
 6. A methodof forming an aperture portion of reactor building as claimed in claim1, wherein a protecting apparatus covering the location where saidaperture portion is to be assembled and having doors which are passableof said structure is provided onto said roof of said reactor buildingprior to forming of said aperture portion.